U.S. patent application number 11/911930 was filed with the patent office on 2008-09-04 for adaptive protection circuit for a power amplifier.
This patent application is currently assigned to Freescale Semiconductor, Inc.. Invention is credited to Walid Karoui, Gilles Montoriol, Philippe Riondet.
Application Number | 20080211585 11/911930 |
Document ID | / |
Family ID | 35094331 |
Filed Date | 2008-09-04 |
United States Patent
Application |
20080211585 |
Kind Code |
A1 |
Karoui; Walid ; et
al. |
September 4, 2008 |
Adaptive Protection Circuit For a Power Amplifier
Abstract
A radio frequency device comprises a radio frequency (RF) power
amplifier (PA) operably coupled to a protection circuit for
minimising voltage standing wave ratio effects, wherein the
protection circuit comprises a current limiter indexed to a power
supplied to the RF PA. In this manner, the protection circuit
combines detection of both current and voltage increase in order to
provide a direct feedback on the final RF PA stage via a bias
control.
Inventors: |
Karoui; Walid; (Toulouse,
FR) ; Montoriol; Gilles; (Tournefeuille, FR) ;
Riondet; Philippe; (Ramonville-St-Ange, FR) |
Correspondence
Address: |
FREESCALE SEMICONDUCTOR, INC.;LAW DEPARTMENT
7700 WEST PARMER LANE MD:TX32/PL02
AUSTIN
TX
78729
US
|
Assignee: |
Freescale Semiconductor,
Inc.
Austin
TX
|
Family ID: |
35094331 |
Appl. No.: |
11/911930 |
Filed: |
April 18, 2005 |
PCT Filed: |
April 18, 2005 |
PCT NO: |
PCT/EP05/05210 |
371 Date: |
October 18, 2007 |
Current U.S.
Class: |
330/298 ;
330/207P |
Current CPC
Class: |
H03F 3/19 20130101; H03F
2200/78 20130101; H03F 2200/451 20130101; H03F 1/52 20130101; H03F
2200/477 20130101; H03F 2200/462 20130101; H03F 1/30 20130101 |
Class at
Publication: |
330/298 ;
330/207.P |
International
Class: |
H03F 1/52 20060101
H03F001/52; H03F 3/04 20060101 H03F003/04 |
Claims
1. A radio frequency device comprises a radio frequency (RF) power
amplifier (PA) operably coupled to a protection circuit for
minimising voltage standing wave ratio effects, wherein the
protection circuit is operably coupled between an input port and a
collector port of the RF PA and comprises a current sense function
for sensing the power amplifier DC current and a current limiter
indexed to a power supplied to the RF PA for limiting the power
amplifier DC current in response to the sensed current.
2. A radio frequency device according to claim 1, wherein the
current limiter is a monolithically integrated current limiter.
3. A radio frequency device according to claim 1, wherein the radio
frequency device is a wireless communication unit and the power
supplied to the RF PA is from a battery.
4. A radio frequency device according to claim 1, further
characterised in that a comparator function is operably coupled to
the current sense function and arranged to compare a threshold
voltage value to a detected voltage coupled to the sensed power
amplifier DC current.
5. A radio frequency device according to claim 4 further comprising
a power amplifier bias circuit operably coupled to the protection
circuit such that when the power amplifier DC current is equal to
or exceeds the threshold voltage, the protection circuit prevents
the power amplifier DC current from increasing by de-biasing power
supplied to the RF power amplifier via the bias circuit.
6. A radio frequency device according to claim 1, further
characterised in that the protection circuitry and the RF PA use
substantially a similar type of active device.
7. A radio frequency device according to claim 5 further
characterised in that the sensed power amplifier DC current relates
to a voltage supplied to the RF PA such that the protection circuit
senses both a current and a supply voltage increase and in response
to the sensing controls a bias level applied to the RF PA.
8. A radio frequency device according to claim 1, wherein the radio
frequency device is a radio frequency (RF) power amplifier (PA)
module.
9. A radio frequency device according to claim 1, wherein the
current sense function comprises a first transistor having a
collector port operably coupled to receive the RF power amplifier
DC current and a base port operably coupled to a base port of the
RF PA.
10. A radio frequency device according to claim 9 further
characterised in that the current sense function further comprises
a sensing resistor operably coupled to an emitter port of the first
transistor to sense the RF power amplifier DC current.
11. A radio frequency device according to claim 9, further
characterised in that the current sense function further comprises
a capacitor arranged in parallel to the sensing resistor.
12. A radio frequency device according to claim 9, further
characterised in that the current sense function further comprises
a second transistor operably coupled to an emitter port of the
first transistor and arranged to limit RF power amplifier DC
current in response to the sensed current.
13. A method of limiting current supplied to a radio frequency (RF)
power amplifier (PA) module operably coupled to a protection
circuit, the method comprising: sensing a power amplifier direct
current (DC) applied to the RF PA in a protection circuit coupled
between an input port and a collector port of the RF PA; comparing,
by the protection circuit, a threshold voltage value to the
detected voltage coupled to the sensed power amplifier DC current;
and limiting, by the protection circuit, direct current applied to
the RF PA if the sensed RF PA direct current equals or exceeds the
threshold value.
14. A method of limiting current supplied to a RF PA module
according to claim 13 wherein the step of limiting comprises
de-biasing current supplied to a base port of the RF power
amplifier transistor.
15. A method of limiting current supplied to a RF PA module
according to claim 13 further characterised in that the step of
sensing a direct current comprises sensing voltage supplied to the
RF PA transistor as a consequence of the protection circuit
topography.
Description
FIELD OF THE INVENTION
[0001] The preferred embodiment of the present invention relates to
a power amplifier (PA) module. The invention is applicable to, but
not limited to, an adaptive protection circuit for a wireless
communication unit's radio frequency power amplifier arranged to
prevent high current effects due to a voltage standing wave ratio
(VSWR).
BACKGROUND OF THE INVENTION
[0002] In the field of radio frequency (RF) power amplifiers (PAs),
a PA is typically designed to be `matched` into a 50 ohm load
impedance, to ensure efficient power transfer from an RF input
signal to an amplified RF output signal. This enables a low power
RF input signal to be amplified and a maximum amount of the
amplified signal forwarded on to, say, an antenna switch and/or an
antenna. In this manner, maximum power transfer is achieved and
minimal power is reflected back into the PA output. The reflection
back of power is typically due to "load mismatch", for example
where the antenna switch or antenna load does not exhibit a 50 Ohm
load. This can be due to the antenna being located near an object
that affects its radiation properties, and correspondingly its
impedance values.
[0003] It has been found that load mismatch problems occur, in
particular, under both high power conditions and when a high
battery voltage is applied to the power amplifier. In this regard,
it can be observed that the DC current increases more than when
operating under normal (50-Ohm load) conditions. The increase of DC
current, under load mismatch conditions, is highly undesirable
(particularly in a hand-portable environment) as it causes
increased power consumption and may over-load the power amplifier
transistor device thereby resulting in damage to, or failure of,
the device.
[0004] Known mechanisms do not solve the aforementioned problems.
For example, voltage limiters do not protect the PA from mismatch
conditions that lead to high current. Furthermore, existing current
limiters do not protect efficiently the PA from battery voltage
variations under mismatch. Known solutions use circuitry external
to the power amplifier module to realize the function, when the PA
is located on Gallium Arsenide (GaAs). Undesirably, this results in
extra inputs/outputs (I/O) leads on the PA die. Alternatively, if a
monolithically integrated circuit (IC) is used, for example where
the PA is manufactured on Silicon Germanium (SiGe), approximately
20% of additional die size is required.
[0005] To accommodate mismatch problems, protection circuits are
often used. A standard current limiter protection circuit is
illustrated in FIG. 1. In FIG. 1, the radio frequency input signal
(RF.sub.in) 105 is input to a base port of power transistor 110.
The power transistor 110 is supplied from a battery voltage 115 via
an RF choke inductor 120 to provide a RF amplified output voltage
125.
[0006] The emitter port of the RF power transistor 110 includes a
sensing resistor 130 to ground. The emitter port of the RF power
transistor 110 is also operably coupled to a protection circuit
140. The protection circuit 140 comprises a multiplier circuit 145,
receiving the detected voltage developed across the sensing
resistor 130 and a comparator circuit 150, comparing the output
from the multiplier circuit 145 with a reference voltage 155, which
sets the chosen limiting current. The comparator circuit 150 output
is input to a transistor (Q1), which effectively is switching the
bias circuit 135 to `ground` when the voltage coming from the
multiplier 145 is greater than the reference voltage 155, thereby
reducing the bias current of the power transistor 110.
[0007] Thus, as shown, the use of a protection circuit adds
significantly to the size and complexity of power amplifier
circuits, with regard to the extra circuitry/components (typically
two operational amplifiers) that are required to generate/compare
the detected voltage to the reference voltage 155, which is set by
additional external circuitry.
[0008] However, the effectiveness of the protection circuit is
compromised due to the operational amplifier offset voltage
.epsilon.1 (comparable to the detected voltage V.sub.det) and to
the variation of V.sub.det with regard to sensing resistor 130
dispersion. It is noteworthy that a low value resistor is more
sensitive to process variation than a high value resistor. In
addition, the collector efficiency of the power transistor 110 is
degraded by the sensing resistor 130. Furthermore, the complexity
of the protection circuit may typically lead to undesirable loop
stability issues, due to high loop gain.
[0009] Thus, a need exists for an improved RF device, such as a
wireless communication unit, RF PA module and method of operation
therefor, which prevents high current under extreme VSWR
conditions, wherein the aforementioned problems with prior art
arrangements are substantially alleviated.
STATEMENT OF INVENTION
[0010] In accordance with aspects of the present invention, there
is provided a radio frequency device, a PA module and method of
operation that prevents high current under VSWR and high battery
voltage, as defined in the appended Claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 illustrates a known current limiter implementation
using a standard protection circuit design.
[0012] Exemplary embodiments of the present invention will now be
described, by way of example only, with reference to the
accompanying drawings, in which:
[0013] FIG. 2 illustrates a wireless communication unit adapted in
accordance with the preferred embodiment of the present
invention;
[0014] FIG. 3 illustrates a current limiter implementation of a
protection circuit, adapted in accordance with the preferred
embodiment of the present invention;
[0015] FIG. 4 illustrates a graphical comparison of a PA
performance with and without the protection circuit according to
the preferred embodiment of the present invention;
[0016] FIG. 5 illustrates graphically a collector current versus
phase at a voltage standing wave ratio (VSWR)=10:1, illustrating
the advantages provided by the preferred embodiment of the present
invention; and
[0017] FIG. 6 illustrates a flowchart of the current limiting
process applied by the protection circuit to a RF PA in accordance
with the preferred embodiment of the present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0018] The preferred embodiment of the present invention will be
described with respect to an implementation of a radio frequency
(RF) power amplifier (PA) module in a wireless communication unit,
such as a mobile phone. However, it will be appreciated by a
skilled artisan that the inventive concept herein described may be
embodied in any type of RF amplifier unit. In summary, the
inventive concept of the present invention proposes a
monolithically integrated PA module having a cost and size
efficient current limiter, notably indexed to battery voltage, for
improving a Power Amplifier's (PA) ruggedness, for example, under
extreme VSWR conditions over battery voltage range.
[0019] In a mobile context, in the known prior art, the current
threshold or current limit is fixed, as a reference voltage is used
for the battery. However, the battery voltage varies. Thus, and as
addressed by the inventive concept hereinafter described, it is
important to have a current threshold or current limit that varies
as the battery voltage drops. In the present specification, this
relationship is termed `indexing`, for example the current
threshold varies with regard to battery voltage.
[0020] Referring now to FIG. 2, a block diagram of a wireless
communication unit 200, capable of supporting the inventive concept
of the preferred embodiment of the present invention, is
illustrated. For the sake of clarity, the wireless communication
unit 200 is shown as divided into two distinct portions--a receiver
chain 210 (which will be described briefly for completeness) and a
transmit chain 220.
[0021] The wireless communication unit 200 contains an antenna
preferably coupled to an antenna switch 204 that provides signal
control of radio frequency (RF) signals in the wireless
communication unit 200, as well as isolation, between the receiver
chain 210 and transmit chain 220. Clearly, the antenna switch 204
could be replaced with a duplex filter, for frequency duplex
communication units, as known to those skilled in the art.
[0022] For completeness, the receiver chain 210 of the wireless
communication unit 200 will be briefly described. The receiver
chain 210 includes a receiver front-end circuit 206 (effectively
providing reception, filtering and intermediate or base-band
frequency conversion). The receiver front-end circuit 206 is
serially coupled to a signal processing function (generally
realised by at least one digital signal processor (DSP)) 208. A
controller 214 is operably coupled to the front-end circuit 206 so
that the receiver is able to calculate receive bit-error-rate (BER)
or frame-error-rate (FER) or similar link-quality measurement data
from recovered information via a received signal strength
indication (RSSI) 212 function. The RSSI 212 function is operably
coupled to the receiver front-end circuit 206. The memory device
216 preferably stores a wide array of data, such as
decoding/encoding functions and the like, as well as amplitude and
phase settings to ensure a linear and stable output. A timer 218 is
operably coupled to the controller 214 to control the timing of
operations, namely the transmission or reception of time-dependent
signals.
[0023] As regards the transmit chain 220, this essentially includes
a signal processor 228, operably coupled to a modulation and
up-converter circuit 222 and power amplifier module 224. The
processor 228 and modulation and up-converter circuit 222 are
operationally responsive to the controller 214. The power amplifier
module 224 is operably coupled to a protection circuit 226, adapted
in accordance with the preferred embodiment of the present
invention.
[0024] The power amplifier module and protection circuit, adapted
in accordance with the preferred embodiment of the present
invention, is further illustrated in FIG. 3. Referring now to FIG.
3, a radio frequency input signal (RF.sub.in) 305 is input to a
base port of power amplifier transistor 224. The power amplifier
transistor 224 is supplied by a battery voltage 315 via a RF choke
inductor 320 to provide a RF amplified output voltage 325. Notably,
the emitter port of the RF power transistor 224 is grounded.
[0025] In particular, the output 325 of the RF power transistor 224
is operably coupled to a protection circuit 345. The protection
circuit 226 is integrated onto the PA die and comprises a first
transistor 355, whose collector port is operably coupled to the
output 325 and whose base port 350 is operably coupled to the base
port of the RF power transistor 224. The emitter port of the first
transistor 355 of the protection circuit 226 is operably coupled to
a parallel resistor 365-capacitor 370 (R-C) circuit coupled to
ground and a base port of a second transistor 360. The collector
380 of the second transistor 360 is coupled to the bias circuit
335, which is operably coupled to the base port of the RF power
transistor 224.
[0026] Under extreme VSWR conditions, and high battery voltage, the
collector current of the power transistor 224 increases more than
would be expected under a 50-ohm load. The current flowing through
the elementary transistor 355 increases proportionally to the
current in the power transistor 224. The detected voltage developed
across the sensing resistor 365 also increases with the
above-mentioned current. The function of the parallel capacitor 370
is arranged to low pass filter the detected voltage V.sub.det.
Then, this detected DC voltage is input to the base of the second
elementary transistor 360. When the detected DC voltage V.sub.det
exceeds the turn-on voltage V.sub.beon of the second elementary
transistor 360, it switches the bias circuit 335 to ground, hence
reducing the bias current of the power transistor 224.
[0027] In the most common case of a multi-stage power amplifier,
when the battery voltage increases, the RF input signal RF.sub.in
305 on the base of the power transistor 224 (as well as on the base
of the first elementary transistor 355) increases leading to a
higher current flowing through the sensing resistor 365. This
consequently leads to a higher detected voltage V.sub.det. The
turn-on voltage of the second elementary transistor 360 is then
reached for a lower collector current in the first elementary
transistor 355, and therefore for a lower collector current in the
power transistor 224.
[0028] In this manner, the inventive concept proposes a
monolithically integrated current limiter protection circuit 226,
which is indexed to the battery voltage 315, for improving the PA's
ruggedness. The power amplifier DC current, dependent upon the
battery voltage 315, is thus sensed, transformed into voltage
V.sub.det across a high value sensing resistor 365 and compared to
a threshold voltage value (set by the turn-on voltage of the
secondary transistor 360). In this manner, the threshold voltage
value is compared to the detected voltage coupled to the sensed
power amplifier DC current. When the detected voltage exceeds this
threshold value, the DC current is prevented from increasing to an
undesirable level by de-biasing the power amplifier.
[0029] Thus, the sensed current is indexed to battery voltage 315
for the protection circuit 226. Furthermore, and preferably, the
protection circuit 226 uses the same type of device as the RF
transistor 224 to be protected. Advantageously, this makes it very
easy to integrate into a monolithic IC, thereby removing extra
input/output pins between dies.
[0030] Moreover, the protection circuit 226 advantageously
comprises a low component count, thereby facilitating a reduced
size for the protected PA system.
[0031] On one hand, the improved protection circuit 226 does not
degrade the RF PA performance on a 50-Ohm load, as illustrated in
FIG. 4. Under such conditions, the protection circuit 226 is `OFF`
and its current consumption is negligible when compared to the
power amplifier 224 collector current. On the other hand, the low
current in the first elementary transistor 355 allows use of a high
value resistor 365, which is advantageously less sensitive to
process variation.
[0032] Elementary transistor (incorporates sensing resistor) is a
small version of the power transistor.
[0033] This solves the prior art problem of a high current flowing
through the power amplifier 110 in FIG. 1, where only a low value
sensing resistor 130 can be used in order to minimize PA
performance degradation.
[0034] Referring now to FIG. 4, a graphical comparison 400 of a PA
performance with and without the improved protection circuit 226 is
illustrated. As shown, for the same input RF power level
(RF.sub.in) 410, the same gain 405, 420 and same RF power output
level 415, 425 can be achieved by the RF power amplifier transistor
224 employing the inventive concept hereinbefore described as well
as an RF power amplifier transistor 224 that is not configured to
employ the inventive concept.
[0035] Referring now to FIG. 5, a collector current 505 versus
phase 510 at a voltage standing wave ratio (VSWR) of 10:1 is
illustrated graphically 500. The graph 500 highlights the current
limiting associated with employing the preferred embodiment of the
present invention. The graph 500 illustrates a collector current
515 having a peak collector current of approximately 3.5 A at a
phase of 120 degrees when the circuit is not configured with the
aforementioned improved protection circuit 226.
[0036] When employing the improved protection circuit 226 of the
inventive concept hereinbefore described, the collector current of
the power transistor 226 does not exceed 2.5 A. This limit is
selected by setting the appropriate value of sensing resistor
365.
[0037] Referring now to FIG. 6, a flowchart 600 illustrates an
overview of the current limiting protection process according to
the preferred embodiment of the present invention. The RF input
power is applied to the RF PA and as the RF PA is turned on in step
605, the DC current drawn by the RF PA is sensed in step 610. The
sensed DC current is transformed into a sensed DC voltage and then
compared with a threshold value by the protection circuit in step
615. Thus, the threshold voltage value is compared to the detected
voltage coupled to the sensed power amplifier DC current. If the
sensed DC voltage does not exceed the threshold value in step 620,
limitation on the collector current does not occur.
[0038] However, if the sensed DC voltage exceeds the threshold
value, in step 620, the protection circuit limits the current being
drawn by the RF PA by de-biasing the PA, as shown in step 625. The
sensing process then continues in step 610, to determine whether
further current limiting needs to be performed by further
de-biasing of the PA.
[0039] A skilled artisan will appreciate that in other
applications, alternative functions/circuits/devices and/or other
techniques may be used. Although the preferred embodiment of the
present invention has been described with respect to a mobile
communication application addressing high current effects due to
VSWR effects, it is envisaged that the inventive concept is equally
applicable to any application that detects output power and uses
the sensed current information for any other purpose, such as
linearisation techniques. Furthermore, it is envisaged that other
R-C topographies may be used, that incorporate a sensing
resistor.
[0040] It is within the contemplation of the present invention that
the inventive concept can be applied to any product that uses RF
power amplifiers, regardless of the frequency or associated
technology of the product. It is also envisaged that the inventive
concept is equally applicable to use with any RF power transistor,
irrespective of the technology, such as bipolar transistors or
heterojunction bipolar transistors (HBTs).
[0041] It will be understood that the improved RF device, for
example a wireless communication unit such as a mobile phone, a RF
PA module and method of operation therefore, which prevents high
current being drawn by the RF PA under extreme VSWR conditions, as
described above, aims to provide at least one or more of the
following advantages: [0042] (i) There is no need to detect the
VSWR effects on the RF power amplifier transistor. [0043] (ii) The
inventive concept combines detection of both current and supply
voltage increase. Since mainly high collector current leads to
power transistor permanent failure, the inventive concept limits
the collector current and therefore maintains this current into non
destructive operating values [0044] (iii) The inventive concept
provides a direct feedback on the final RF power amplifier stage
itself, via the DC bias circuit. [0045] (iv) The inventive concept
is very easy to implement, in that the improved protection circuit
has a low component count. For example, the improved protection
circuit requires only two transistors and does not require use of
any operational amplifier. This facilitates a smaller die size.
[0046] (v) The inventive concept is supply (battery) voltage
dependent. [0047] (vi) The inventive concept provides low
sensitivity to process variations, due to the use of a high value
resistor in the protection circuit. [0048] (vii) The inventive
concept can be tuned for any collector current limit value. [0049]
(viii) The inventive concept can be integrated on GaAs with no
additional I/O pins required.
[0050] In particular, it is envisaged that the aforementioned
inventive concept can be applied by a semiconductor manufacturer to
any power amplifier module having a protection circuit. It is
further envisaged that, for example, a semiconductor manufacturer
may employ the inventive concept in a design of a stand-alone
device, such as a RF power amplifier module, or
application-specific integrated circuit (ASIC) and/or any other
sub-system element.
[0051] Whilst the specific and preferred implementations of the
embodiments of the present invention are described above, it is
clear that one skilled in the art could readily apply variations
and modifications of such inventive concepts.
[0052] Thus, an improved RF device such as a wireless communication
unit, RF PA module and method of operation therefor have been
described, particularly to prevent a high current being drawn by
the RF PA under extreme VSWR conditions, wherein the aforementioned
disadvantages with prior art arrangements have been substantially
alleviated.
* * * * *